Why is yahoo messenger for iphone not working?

I am not able to log in to yahoo messenger. I had the problem for several months on my iPhone 4S and now on my iPhone 5S I'm experiencing the identical problem. I am using ios 8.0. I get a message "Sign in problem, Unable to login right now, please try again later." I have uninstalled and reinstalled several times to no avail. I have no problem logging onto PC yahoo messenger version. The ap looks like it will log on when I first access it and then it fails to that error message. The rating for yahoo messenger in the ap store is terrible so I suspect this is a systemic problem.

Any advice would be appreciated.